Owing to the recent technical advancements of image processing by a computer, storage of a large amount of data and data communication, input of information, revision, edition, layout, and pagination are consistently computerized, and electronic editorial system enabling instantaneous output on a remote terminal plotter through a high speed communication network or a communications satellite has been practically used.
Light-sensitive materials having high photo-sensitivity which may provide direct type printing plate precursors directly preparing printing plates based on the output from a terminal plotter include electrophotographic light-sensitive materials.
In order to form a lithographic printing plate using an electrophotographic light-sensitive material, a method wherein after the formation of toner image by an electrophotographic process, non-image areas are subjected to oil-desensitization with an oil-desensitizing solution to obtain a lithographic printing plate, and a method wherein after the formation of toner image, a photoconductive layer is removed in non-image areas to obtain a lithographic printing plate are known.
However, in these method, since the light-sensitive layer is subjected to treatment for rendering it hydrophilic to form hydrophilic non-image areas or removed by dissolving out it in the non-image areas to expose an underlying hydrophilic surface of support, there are various restrictions on the light-sensitive material, particularly a photoconductive compound and a binder resin employed in the photoconductive layer. Further, printing plates obtained have several problems on their image qualities or durability.
In order to solve these problems there is proposed a method comprising providing a transfer layer composed of a thermoplastic resin capable of being removed upon a chemical reaction treatment on a surface of an electrophotographic light-sensitive element, forming a toner image on the transfer layer by a conventional electrophotographic process, transferring the toner image together with the transfer layer onto a receiving material capable of forming a hydrophilic surface suitable for a lithographic printing, and removing the transfer layer to leave the toner image on the receiving material whereby a lithographic printing plate is prepared as described in WO 93/16418.
Since the method for preparation of printing plate using a transfer layer is different from the method for forming hydrophilic non-image areas by modification of the surface of light-sensitive layer or dissolution of the light-sensitive layer, and comprises the formation of toner image not on the light-sensitive layer but on the transfer layer, the transfer of toner image together with the transfer layer onto another support having a hydrophilic surface and the removal of the transfer layer by a chemical reaction treatment, printing plates-having good image qualities are obtained without various restrictions on the photoconductive layer employed as described above.
However, in the above-described method, transferability of the transfer layer while applying heat and pressure is yet insufficient and thus, there are observed lack of fine images on the receiving material and the residue of toner image and transfer layer on the surface of light-sensitive element in some cases. In particular, a support having a hydrophilic surface to be used as the receiving material is restricted in order to obtain good transferability of transfer layer. Specifically, in case of employing a receiving material comprising a substrate having a surface of relatively poor smoothness, adhesion of the transfer layer to the receiving material is insufficient and as a result, transferability decreases. Further, the transfer layer must fulfill electrophotographic characteristics (Ep characteristics) in addition to the transferability and a dissolution property which is important in the step of preparing a printing plate, because on the transfer layer provided on a light-sensitive element are formed toner images by a conventional electrophotographic process.
It is not easy to select a transfer layer which satisfies all of the transferability, dissolution property and electrophotographic characteristics. Accordingly, a resin to be employed in the transfer layer is imposed various restrictions on its basic structure such as polymer component and molecular weight.
The electrophotographic characteristics, particularly, chargeability and dark decay (DQR) of transfer layer are greatly influenced by properties of resin used. In the event of poor electrophotographic characteristics, problems on image reproduction, for example, decrease in the maximum density of duplicated image and lack of fine lines and letters may tend to occur. Such a tendency becomes large when a thickness of the transfer layer is more than 5 .mu.m. To reduce the thickness of transfer layer, however, may result in degradation of transferability. Therefore, it is very difficult to satisfy both of the electrophotographic characteristics and the transferability.